Antennas for wireless devices have to be small, which implies restrictions on the bandwidth. There is a well known trade-off between antenna size and bandwidth. The smaller the antenna, the smaller the bandwidth, and particularly, typical prior-art internal antennas for a handheld device feature a 5-15% relative bandwidth at frequencies such as those of typical cellular, mobile and wireless services (800 MHz-2200 MHz). When an internal antenna is operated outside its operating bandwidth, the gain, the efficiency and matching characteristics (VSWR, return-loss) of the antenna become severely degraded to unacceptable levels.
There are antenna systems for wireless devices featuring a wide bandwidth. Those antenna systems rely substantially on the radiating efficiency of the ground-plane or on a large antenna element, and are very sensitive to hand loading effects.
For instance, in an antenna system with a single large antenna element, when the user is operating the wireless device, the proximity of the hand to this large antenna element (in which the currents are high compared to the currents in the ground plane) facilitates an electrical coupling between the hand and the antenna element which may detune the antenna element, may change its impedance, and may in addition cause radiation losses.
It has been observed that known prior art solutions feature antenna elements typically located at the ends of the wireless handheld device. Thus, when the wireless handheld device is being operated, the hand does not shield/cover the antenna element and, therefore, the hand loading effects are minimized. There is a very obvious trade-off, since when the antenna element is made bigger to achieve a wide bandwidth, the hand loading effects increase as the area used by the antenna system increases.
As stated earlier, there are also prior art antenna systems featuring a wide bandwidth which rely largely on the radiating efficiency of the ground-plane. Since handheld devices feature a ground-plane which typically extends throughout the whole device, the antenna system ideally featuring a wide bandwidth becomes very sensitive to hand loading effects. As a result of the hand loading effects, the bandwidth is reduced or the whole antenna system may be detuned.
One of the challenges that antenna designers face is providing an antenna system for a handset that features a wide bandwidth while not being substantially influenced by hand loading. As previously stated, while reducing the size might provide a solution to the problem involved with the hand loading effects, the bandwidth and gain degradations introduced by the size reduction are usually unacceptable.
A particular technique of balancing antennas that minimizes the effect of hand loading is found in B S Collins, S P Kingsley, J M Ide, S A Saario, R W Schlub, and S G O'Keefe, “A multi-band hybrid balanced antenna”, presented at the 2006 IEEE International Workshop on Antenna Technology: Small Antennas and Novel Metamaterials, White Plains, N.Y., Mar. 6-8, 2006. The paper describes a multi-band antenna incorporating a balanced feed network which shows substantial immunity to the usual ground-plane hand loading effects. Said technique is not satisfactory though, since the resulting antenna system has typically twice the size of the original antenna system.
There are several prior-art antennas, as described in patent application publ. no. WO-A-02/065583, entitled “Magnetic dipole and shielded spiral sheet antennas structures and methods”, whose configuration and shape provide a shield to block radio frequency energy from being absorbed in a body. An antenna structure as disclosed in WO-A-02/065583 is designed so that radio frequency energy tends to flow in the direction away from a person. Those prior-art antennas allegedly feature a robust behavior to hand proximity influence, but their bandwidth appears to be insufficient for many practical applications.
Antenna combinations or multiple antenna sets, that is, antenna systems having at least two individual antenna elements whose output signals are combined, are generally known (for instance, Multiple-Input Multiple-Output MIMO and diversity systems).
Antenna diversity is a transmission technique useful in multipath environments. A multipath environment is one where the information-carrying signal is transmitted along different propagation paths. These propagation paths may experience different channel conditions (e.g., different fading, multipath, and interference effects) and may feature different signal-to-noise-and-interference ratios (SNRs). Therefore, the information-carrying signal can arrive through different paths and have different levels. Antenna designers configure diversity antenna systems in such a way that the two or more antenna elements are placed so that they are uncorrelated and, therefore, the information-carrying signal arriving to each one of them will not simultaneously feature minimum levels. A diversity combining circuit combines or selects the signals from the receiver antenna elements to constitute an improved quality signal.
Also, antenna arrays are used when the radiation characteristics required for a certain application are not achievable by a single antenna element. The arrangement of the antenna elements forming an array is normally so that the antenna array features a directive radiation pattern that provides a radiation maximum in a particular direction. Antenna arrays are typically used to achieve directivity in one or more orthogonal polarizations. In such antenna arrays, the distance between the antenna elements is usually larger than half of a free space operating wavelength of the antenna elements, and quite often substantially close to one free space operating wavelength (such as in the order of 0.8 or 0.9 wavelengths, or alike).